GetChar and GetDouble: нет определения; предполагается "extern" с возвратом типa int

[Anonymous_ghost]

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
double first(double a);
double second(double b);
double final(double num1, double num2);

int main(int argc, const char * argv[])
{

	char repeat;
	do {
		double x;
		double x1;
		double y;
		printf("Hello! This program does some math.\nPlease follow the instructions bellow and everything will be great! \n");
		do {
			printf("Please enter a value for x : \n");
			x = GetDouble();
		} while (x < 3);
		x1 = x;
		x = first(x);
		printf("a is %f\n", x);
		x1 = second(x1);
		printf("b is %f\n", x1);
		y = final(x, x1);
		printf("The result is y = %f\n", y);
		printf("Would you like to continue?\n Type y or n\n ");
		repeat = GetChar();
	} while ((repeat == 'y') || (repeat == 'Y'));
}
double first(double a)
{
	a = a * a + log(a);
	return a;
}
double second(double b)
{
	b = sqrt(b + b * b - 10);
	return b;
}
double final(double num1, double num2)
{
	double result;
	if (num1 > -5)
		result = num1 / 2 + (num1 + num2) / 2;
	else
		result = num1 / 2 + 1 / 2 * (num1 + num2);
	return result;
}

[p51x]

Вы не описали и не реализовали две функции... вопрос в чем?